1. Field of the Invention
The present invention relates to an image forming apparatus which employs an electrophotographic system and a tandem system.
2. Description of Related Art
In the electrophotographic system, a charged image supporter is irradiated with modulated light, whereby a latent image is formed. This latent image is developed with a toner, whereby a toner image is formed. The toner image is transferred from the image supporter to a transfer medium.
In the tandem system, image supporters are provided in a casing for respective ones of a plurality of colors (e.g., Y (yellow), M (magenta), C (cyan), K (black)). These image supporters are arranged in parallel along a predetermined carrying direction. On these image supporters, toner images of corresponding colors are concurrently formed. The toner images are transferred to an intermediate transfer medium such that the toner images are superposed on one another, whereby a full-color composite toner image is formed. The composite toner image, which is supported on the intermediate transfer medium, is carried in the carrying direction.
In an image forming apparatus which employs both of the above-described systems, toners and the like remain on the surfaces of the image supporters after transfer of the toner images from the image supporters to the intermediate transfer medium. The image forming apparatus includes a cleaning member configured to remove the transfer residual toners and other deposits from the image supporter surfaces. The cleaning member is, for example, a cleaning blade which is configured to touch the surface of the image supporters and mechanically remove the transfer residual toners and the like.
In recent years, to reduce the downtime of the image forming apparatus, extension of the life of the image supporters and the cleaning member (hereinafter, “the image supporters and other members”) has been demanded. A bottleneck in the life extension is the abrasion loss of the image supporters and other members. The image supporters and other members are to be replaced by new ones when the abrasion loss exceeds a predetermined threshold. Therefore, the life extension requires suppressing abrasion of the image supporters and other members. To this end, the technique of forming a coating of a solid lubricant (hereinafter, “lubricant”) over the surface of the image supporters has been put into practice. According to this technique, smoothness is given to the image supporter surface, so that the frictional force between the image supporter surface and the cleaning member is reduced, whereby abrasion of these components is suppressed.
However, the lubricant coating is abraded when the transfer residual toner reaches a gap between the image supporters and the cleaning member. Therefore, the thickness of the lubricant coating is different between an image portion in which the toner adheres to the image supporter surface and a non-image portion in which no toner adheres. Due to the variation in thickness of the lubricant coating, conventional image forming apparatuses have deteriorated image quality. A specific example of this problem is described below.
In the image forming apparatus, if transfer residual toners adhere and fix to a thin portion of the lubricant coating during continuous printing of a plurality of copies, the transfer residual toners sometimes cannot be removed from the image supporter surface even using the cleaning member. In this case, a potential variation would not occur even if the toner-fixed portion is irradiated with modulated light in a subsequent exposure process. As a result, in some cases, white spot noise is produced on the copies.
In image forming apparatuses for office use which are rarely used for continuous printing of a large number of copies of the same material, the above-described deterioration in the image quality has not been considered as a critical problem. However, this has been considered as a major problem in the field of industrial printing because a large number of copies of the same material are continuously printed and/or the required image quality level is high.
In view of the above problem, Japanese Patent Laid-Open Publication No. 2009-58732 discloses the technique of applying a lubricant to an image supporter while controlling exposure and a bias voltage which is to be applied to a lubricant application brush according to image formation history information. Meanwhile, Japanese Patent Laid-Open Publication No. 2008-225240 discloses the technique of forcibly consuming toners according to the number of printed pixels in the main-scanning direction of the image supporter.
However, in an image forming apparatus which employs the electrophotographic and tandem systems, toners are reversely transferred via an intermediate transfer medium as well known in the art. The reverse transfer means that, relative to a certain image supporter, a toner transferred from an image supporter on the upstream side of a carrying direction of a toner image to an intermediate transfer medium is transferred to a surface of an image supporter on the downstream side. Due to the reversely transferred toner, the amount of toner that reaches a gap between the image supporter and the cleaning member increases. However, since the increase of the toner amount which is attributed to the reversely transferred toner is not considered in the techniques disclosed in Japanese Patent Laid-Open Publications Nos. 2009-58732 and 2008-225240, there is a probability that a relatively large amount of toner adheres to the image supporter surface. As a result, there is a problem that the toner fixes onto the lubricant coating so that the image quality can deteriorate.
In view of the above, an object of the present invention is to provide an image forming apparatus that is capable of suppressing deterioration of the image quality which is attributed to a reversely transferred toner.